Power on reset circuit

ABSTRACT

In a power on reset (POR) circuit, when power is turned on, an output signal of an inverter attains an H level and an N channel MOS transistor is rendered conductive. The potential of an input node of the inverter becomes a potential of a power supply voltage divided by a conductive resistance value R1 of a P channel MOS transistor and a conductive resistance value R2 of an N channel MOS transistor. Assuming that the threshold voltage of the inverter is 0.8 V and R1:R2=2:3, then the power supply voltage Vres at the time when signal POR# inverts its level becomes 1.33 V. Thus, this POR circuit can reliably be utilized even in a product designed to operate with 1.5 V incorporating a MOS transistor having a threshold voltage of 0.8 V.

This application is a divisional of application Ser. No. 10/268,687filed Oct. 11, 2002,now U.S. Pat. No. 6,556,058, which is a divisionalof application Ser. No. 09/784,142, filed Feb. 16, 2001 now U.S. Pat.No. 6,469,552.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power on reset circuits, and moreparticularly, to a power on reset circuit that is incorporated in asemiconductor device and generates a reset signal for resetting thesemiconductor device at the time of power on.

2. Description of the Background Art

Conventionally, a semiconductor integrated circuit device (for example,DRAM, SRAM) is provided with a power on reset circuit (hereinafter,referred to as a “POR circuit”) for resetting an internal circuit whenan external power supply voltage VDD is turned on.

An output signal POR# of POR circuit remains at an L level untilexternal power supply voltage VDD is raised from 0 V to a prescribedvoltage Vres. When external power supply voltage VDD exceeds Vres,output signal POR# attains an H level. Voltage Vres is set lower than acertain range of the power supply voltage with which a product isguaranteed to normally operate. Herein, such a range is called a“guaranteed range”. For example, if a product is designed to operatewith 3.3 V (hereinafter, such product is referred to as a “3.3 Vproduct”), the guaranteed range of the power supply voltage is normallyfrom 3.0 V to 3.6 V. Thus, Vres is set approximately at 2.5 V in thiscase. During a time period in which power supply voltage VDD is notgreater than Vres and signal POR# is at an L level, the internalcircuitry of the semiconductor integrated circuit device, or morespecifically, a redundant circuit of a memory device, a register orstate machine of every kind, is initialized.

In the semiconductor integrated circuit device, in association withminiaturization of MOS transistors, the power supply voltage has beendownscaled from initial 5 V to 3.3 V or to 2.5 V, further to 1.8 V or to1.5 V. Consequently, Vres of POR circuit has also been downscaled.

FIG. 9 is a circuit diagram showing a configuration of such POR circuit30, which is similar to the one disclosed in U.S. Pat. No. 5,703,510.

Referring to FIG. 9, POR circuit 30 includes a P channel MOS transistor31, an N channel MOS transistor 32, capacitors 33, 34, and CMOSinverters 35-37. P channel MOS transistor 31 is connected between a lineof power supply potential VDD and a node N1, and has its gate connectedto node N1. P channel MOS transistor 31 constitutes a diode element. Nchannel MOS transistor 32 is connected between node N1 and a line ofground potential GND, and has its gate connected to a line of powersupply potential VDD. N channel MOS transistor 32 constitutes aresistance element of high resistance. Capacitor 33 is connected betweennode N1 and a line of ground potential GND.

Inverter 35 includes a P channel MOS transistor 38 and an N channel MOStransistor 39. P channel MOS transistor 38 is connected between a lineof power supply potential VDD and a node N2, and has its gate connectedto node N1. N channel MOS transistor 39 is connected between node N2 anda line of ground potential GND, and has its gate connected to node N1.

Inverter 36 includes a P channel MOS transistor 40 and an N channel MOStransistor 41. P channel MOS transistor 40 is connected between a lineof power supply potential VDD and node N1, and its gate is connected tonode N2. N channel MOS transistor 41 is connected between node N1 and aline of ground potential GND, and its gate is connected to node N2.Inverters 35 and 36 constitute a latch circuit.

Capacitor 34 is connected between a line of power supply potential VDDand node N2. Node N2 is connected to an input node of inverter 37. Anoutput signal of inverter 37 becomes signal POR#.

Hereinafter, Vres of POR circuit 30 will be described. In this PORcircuit 30, to obtain Vres lower than that would be obtained by the PORcircuit disclosed in the above-mentioned U.S. Pat. No. 5,703,510, thediode element (P channel MOS transistor 31) connected between the lineof power supply potential VDD and node N1 is reduced from the two stagesto one stage, and at the same time, the threshold voltage VTC ofinverter 35 is reduced to the level of the threshold voltage VTN of Nchannel MOS transistor 39.

More specifically, threshold voltage VTC of CMOS inverter 35 isexpressed as follows: $\begin{matrix}{{VTC} = \quad \frac{{VDD} + {VTP} + {{VTN}\sqrt{B_{R}}}}{1 + \sqrt{B_{R}}}} \\{= \quad \frac{\frac{{VDD} + {VTP}}{\sqrt{B_{R}}} + {VTN}}{\frac{1}{\sqrt{B_{R}}} + 1}}\end{matrix}$

wherein VTP is a threshold voltage of P channel MOS transistor 38; β_(R)represents a ratio β_(N)/β_(P) between β_(N) of N channel MOS transistor39 and β_(P) of P channel MOS transistor 38. β_(N) represents a ratioW_(N)/L_(N) of a gate width W_(N) to a gate length L_(N) of N channelMOS transistor 39, and β_(P) represents a ratio W_(P)/L_(P) of a gatewidth W_(P) to a gate length L_(P) of P channel MOS transistor 38. Thus,by adjusting β_(N)=W_(N)/L_(N) and β_(P)=W_(P)/L_(P), it is possible tomake β_(R)=β_(N)/β_(P) larger than 1, whereby VTC nearly equal to VTN isattained.

If node N1 is at an L level, P channel MOS transistor 40 of inverter 36is rendered non-conductive, and N channel MOS transistor 41 isconductive. If β_(N) of N channel MOS transistor 41 is made sufficientlysmall, potential V1 of node N1 becomes approximately equal to VDD−VTP,wherein VTP represents a threshold voltage of P channel MOS transistor40.

If potential V1 of node N1 exceeds threshold potential VTN of inverter35, potential V1 of node N1 inverts from an L level to an H level. Thus,power supply voltage VDD at the time when potential V1 of node N1 risesfrom an L level to an H level, i.e., Vres, becomes equal to VTN+VTP.

FIG. 10 shows time charts illustrating the operation of POR circuit 30shown in FIG. 9. Referring to FIG. 10, at the initial state, node N1 isat a ground potential GND since it is grounded through a resistanceelement (N channel MOS transistor 32) of high resistance. Assume thatexternal power supply potential VDD is switched on at time t0 and powersupply potential VDD rises towards 1.8 V in proportion to time. Whenpotential VDD>VTP, the diode element (N channel MOS transistor 31) turnson, and potential V1 of node N1 becomes equal to VDD−VTP.

At time t1, when potential V1 (=VDD−VTP) of node N1 exceeds thresholdpotential VTN of inverter 35, the output level of inverter 35 invertsfrom an H level to an L level, and the output level of inverter 36 risesfrom an L level to an H level, so that potential V1 of node N1 risesfrom VDD−VTP to VDD. Power supply voltage VDD at this time is Vres, andVres=VTN+VTP in this POR circuit 30. Therefore, signal POR# is at an Llevel from time t0 to time t1, and it rises to an H level at time t1.Even if power supply voltage VDD fluctuates in a range higher than VTNafterwards, V1=VDD, and thus, signal POR# remains at the H level (timet1-t7). When power supply voltage VDD drops lower than VTN (time t8),MOS transistors 31, 38, 39, 40, 41 are rendered non-conductive. Electriccharges stored in capacitor 33 are discharged through the resistanceelement (N channel MOS transistor 32) of high resistance, and PORcircuit 30 returns to its initial state.

When power supply voltage VDD of a semiconductor integrate circuitdevice is downscaled, the threshold voltage of a MOS transistor shouldbe reduced correspondingly. In practice, however, to lower powerconsumption by restricting a leakage current, the threshold voltage ofMOS transistor is not downscaled. More specifically, the thresholdvoltage of MOS transistor, which was 0.8 V for 5 V and 3 V products, ismaintained at 0.8 V even for 1.8 V and 1.5 V products. Thus, Vres of thePOR circuit 30 shown in FIG. 9 becomes equal to VTN+VTP=0.8+0.8=1.6 V.

The guaranteed range of the power supply voltage for a 1.8 V product is1.62 V to 1.98 V. Thus, the margin guaranteed by Vres=1.6 V as above isnot large enough. Further, POR circuit 30 of FIG. 9 cannot be used for a1.5 V product.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a power onreset circuit that can be used even in a low power consumptionsemiconductor device operative with low power supply voltage.

The power on reset circuit according to the present invention includes:an inverter that drives a reset signal to an activated level in responseto reception of a power supply potential and a reference potential anddrives the reset signal to an inactivated level in response to apotential of its input node exceeding a prescribed threshold potential;a first resistance element having one electrode receiving a power supplypotential and the other electrode connected to the input node of theinverter; and a first transistor of a first conductivity type having itsfirst electrode receiving a reference potential and its second electrodeconnected to the input node of the inverter, and rendered conductive inresponse to the reset signal attaining the activated level. Therefore,when power is turned on, a potential of the power supply voltage dividedby a resistance value of the first resistance element and a conductiveresistance value of the first transistor is supplied to the inverter, todrive the reset signal to the activated level. When the dividedpotential exceeds a threshold potential of the inverter, the resetsignal is driven to the inactivated level. Thus, the level of the powersupply voltage at which the reset signal is driven from the activatedlevel to the inactivated level can be set lower than in the conventionalcase, so that even a semiconductor device consuming less power andoperating with less power supply voltage is enabled to generate a resetsignal.

Preferably, the first resistance element includes a second transistor ofa second conductivity type having its first electrode receiving thepower supply potential, its second electrode connected to the input nodeof the inverter, and its input electrode receiving the referencepotential. In this case, the inverter receives a potential of the powersupply voltage divided by the conductive resistance values of the firstand second transistors.

Preferably, the inverter includes: a third transistor of the secondconductivity type having its first electrode receiving the power supplypotential, its second electrode connected to an output node of theinverter, and its input electrode connected to the input node of theinverter; and a fourth transistor of the first conductivity type havingits first electrode receiving the reference potential, its secondelectrode connected to the output node, and its input electrodeconnected to the input node. The predetermined threshold potential isset approximately equal to a threshold potential of the fourthtransistor. In this case, it is possible to set the threshold potentialof the inverter to a lowest possible level.

Preferably, a first capacitor having one electrode receiving thereference potential and the other electrode connected to the input nodeof the inverter, and a second capacitor having one electrode receivingthe power supply potential and the other electrode connected to theoutput node of the inverter are further provided. In this case, it ispossible to stabilize the potentials of the input node and the outputnode of the inverter.

Still preferably, the first capacitor includes a fifth transistor of thefirst conductivity type having its first and second electrodes bothreceiving the reference potential and its input electrode connected tothe input node of the inverter, and the second capacitor includes asixth transistor of the second conductivity type having its first andsecond electrodes both receiving the power supply potential and itsinput electrode connected to the output node of the inverter. In thiscase, the first and second capacitors can readily be constituted.

Preferably, a seventh transistor of the first conductivity type havingits first electrode and its input node receiving the reference potentialand its second electrode connected to the input node of the inverter,and an eighth transistor of the second conductivity type having itsfirst electrode and its input electrode both receiving the power supplypotential and its second electrode connected to the output node of theinverter are further provided. In this case, it is possible to drive thereset signal to the activated level even if the power supply potentialis slowly raised up, thereby preventing malfunction of the semiconductordevice.

Preferably, a second resistance element having one electrode receivingthe reference potential and the other electrode connected to the inputnode of the inverter is further provided. In this case, it is possibleto discharge the charges in the input node of the inverter via thesecond resistance element to the line of the reference potential afterstopping the application of the power supply potential, so that theinput node of the inverter can be driven to the reference potential in ashort period of time.

Still preferably, the second resistance element includes a ninthtransistor of the first conductivity type having its first electrodereceiving the reference potential, its second electrode connected to theinput node of the inverter, and its input electrode receiving the powersupply potential. In this case, it is readily possible to constitute thesecond resistance element.

Preferably, a tenth transistor of the first conductivity type having itsfirst electrode receiving the power supply potential and its secondelectrode connected to the input node of the inverter, a thirdresistance element having one electrode receiving the power supplypotential and the other electrode connected to the input electrode ofthe tenth transistor, and a third capacitor having one electrodereceiving the reference potential and the other electrode connected tothe input electrode of the tenth transistor are further provided. Inthis case, after stopping the application of the power supply potential,charges at the input node of the inverter can be discharged via thefirst transistor, so that the input node of the inverter can be drivento the reference potential in a short period of time.

Still preferably, the third resistance element includes an eleventhtransistor of the second conductivity type having its first electrodereceiving the power supply potential, its second electrode connected tothe input node of the inverter, and its input electrode receiving thereference potential. In this case, the third resistance element canreadily be constituted.

Preferably, a fourth resistance element connected in series with thefirst resistance element between a line of the power supply potentialand the input node of the inverter and having a resistance value that issufficiently larger than the conductive resistance value of the firstresistance element, and a fifth resistance element connected in serieswith the first transistor between a line of the reference potential andthe input node of the inverter and having a resistance valuesufficiently larger than the conductive resistance value of the firsttransistor are further provided. In this case, a potential of the powersupply voltage divided by the fourth and fifth resistance elements isapplied to the inverter, so that it is possible to stabilize thethreshold voltage of the power on reset circuit.

Still preferably, the fourth and fifth resistance elements are made ofthe same material to have the same width, and have their resistancevalues set by their respective lengths. In this case, it is possible tosuppress the variation in the resistance values of the fourth and fifthresistance elements. Thus, the threshold voltage of the power on resetcircuit can further be stabilized.

Still preferably, the fourth and fifth resistance elements are eachformed of a diffusion resistance layer. In this case, it is readilypossible to constitute the fourth and fifth resistance elements.

Still preferably, the fourth and fifth resistance elements are eachformed of a polycrystalline silicon layer. In this case, again, thefourth and fifth resistance elements can be readily constituted.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a configuration of a power on resetcircuit according to an embodiment of the present invention.

FIG. 2 shows time charts illustrating the operation of the power onreset circuit shown in FIG. 1.

FIG. 3 is a circuit diagram showing a modification of the embodiment.

FIG. 4 is a circuit diagram showing another modification of theembodiment.

FIG. 5 is a circuit diagram showing yet another modification of theembodiment.

FIG. 6 is a time chart illustrating effects of the power on resetcircuit shown in FIG. 5.

FIG. 7 is a circuit diagram showing a further modification of theembodiment.

FIG. 8 is a circuit diagram showing yet another modification of theembodiment.

FIG. 9 is a circuit diagram showing a configuration of a conventionalpower on reset circuit.

FIG. 10 shows time charts illustrating the operation of the power onrest circuit shown in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A POR circuit 1 according to an embodiment of the present inventionshown in FIG. 1 will be contrasted with the conventional POR circuitshown in FIG. 9.

POR circuit 1 shown in FIG. 1 is different from POR circuit 30 of FIG. 9in that P channel MOS transistor 31 is replaced with a P channel MOStransistor 2, and inverter 36 is replaced with an N channel MOStransistor 3. P channel MOS transistor 2 is connected between a line ofpower supply potential VDD and node N1, and its gate is grounded. Pchannel MOS transistor 2 constitutes a resistance element. N channel MOStransistor 3 is connected between node N1 and a line of ground potentialGND, and its gate is connected to node N2.

Hereinafter, Vres of POR circuit 1 will be described. The thresholdvoltage VTC of inverter 35 is equal to the threshold voltage VTN (=0.8V) of N channel MOS transistor 39. Thus, when potential V1 of node N1 islower than VTN, node N2 attains an H level, and N channel MOS transistor3 is rendered conductive. P channel MOS transistor 2 has its gategrounded, and thus is conductive. Therefore, potential V1 of node N1becomes a potential of power supply voltage VDD divided by P channel MOStransistor 2 and N channel MOS transistor 3. More specifically, when theconductive resistance values of P channel MOS transistor 2 and N channelMOS transistor 3 are represented as R2 and R3, respectively, thenpotential V1 of node N1 is equal to VDD·R3/(R2+R3).

When potential V1 of node N1 exceeds threshold potential VTN of inverter35, potential V1 of node N1 inverts from an L level to an H level.Therefore, Vres being the power supply voltage VDD at the time whenpotential V1 of node N1 rises from an L level to an H level becomesequal to VTN(R2+R3)/R3. For example, if R2:R3=2:3, thenVres=0.8×5/3=1.33 V. This value is lower than Vres (=1.6 V) of PORcircuit 30 shown in FIG. 9. POR circuit 1 can thus be used in 1.8 V and1.5 V products.

The time charts shown in FIG. 2, illustrating the operation of PORcircuit 1 of FIG. 1, will be contrasted with the charts in FIG. 10.

Referring to FIG. 2, at the initial state, node N1 is at a groundpotential GND since it is grounded via a resistance element (N channelMOS transistor 32) of high resistance. Assume that external power supplypotential VDD is switched on at time to and power supply potential VDDrises towards 1.8 V in proportion to time.

During the time period in which potential V1 of node N1 is lower thanthreshold potential VTN of inverter 35, node N2 is at an H level and Nchannel MOS transistor 3 is conductive. Potential V1 of node N1 becomesa value 3VDD/5, that is power supply potential VDD divided by P channelMOS transistor 2 and N channel MOS transistor 3 (time t0-t1).

When potential V1 (=3VDD/5) of node N1 exceeds threshold potential VTNof inverter 35 at time t1, the output level of inverter 35 inverts froman H level to an L level, and N channel MOS transistor 3 is renderednon-conductive. Potential V1 of node N1 rises from 3VDD/5 (=VTN) to VDD.Power supply voltage VDD at this time is Vres. In this POR circuit 1,Vres=1.33 V. Therefore, signal POR# is at an L level during the timeperiod t0-t1, and it rises to an H level at time t1.

Even if power supply voltage VDD fluctuates in a range higher than VTNafterwards, V1=VDD, and thus, signal POR# remains at the H level (timet1-t7). When power supply voltage VDD drops and becomes lower than VTN(time t8), MOS transistors 2, 3, 38, 39 are rendered non-conductive. Thecharges stored in capacitor 33 are discharged via the highly resistiveresistance element (N channel MOS transistor 32), and POR circuit 1returns to its initial state.

Hereinafter, various modifications of the embodiment as described abovewill be described. In the modification shown in FIG. 3, N channel MOStransistor 32 and capacitors 33, 34 in POR circuit 1 of FIG. 1 arereplaced with a resistance element 4, an N channel MOS transistor 5 anda P channel MOS transistor 6, respectively. Resistance element 4 havinga high resistance value is provided to set the potential V1 of node N1to 0V when power supply potential VDD is lowered to 0V. Resistanceelement 4 is formed of a diffusion resistance layer, a polycrystallinesilicon layer or the like. N channel MOS transistor 5 has its gateconnected to node N1, and its source and drain connected to the line ofground potential GND. P channel MOS transistor 6 has its gate connectedto node N2, and its source and drain connected to the line of powersupply potential VDD. The gate capacitance of N channel MOS transistor 5and P channel MOS transistor 6 is provided to stabilize the potential atnodes N1 and N2, respectively. This modification allows achievement ofthe same effects as of POR circuit 1 of FIG. 1.

With the modification shown in FIG. 3, assume that the gate capacitanceof N channel MOS transistor 5 and P channel MOS transistor 6 is both setsmall. In this case, when power supply potential VDD is slowly raisedup, node N2 quickly attains an L level due to a leakage current of Nchannel MOS transistor 39 and, likewise, node N1 quickly attains an Hlevel due to a leakage current of P channel MOS transistor 2. Thiscauses signal POR# to remain at an L level for only an extremely shortperiod of time, leading to malfunction of the semiconductor integratedcircuit device. On the other hand, if the gate capacitance of N channelMOS transistor 5 and P channel MOS transistor 6 is both increased, itwill result in an increased layout area.

Thus, in another modification shown in FIG. 4, N channel MOS transistor5 and P channel MOS transistor 6 in the POR circuit of FIG. 3 arereplaced with an N channel MOS transistor 7 and a P channel MOStransistor 8, respectively. N channel MOS transistor 7 has its drainconnected to node N1 and its gate and source connected to the line ofground potential GND. P channel MOS transistor 8 has its drain connectedto node N2 and its gate and source connected to the line of power supplypotential VDD. The sizes of MOS transistors 2 and 7 are set such thatthe leakage current of N channel MOS transistor 7 immediately afterpower-on is larger than the leakage current of P channel MOS transistor2. Further, the sizes of MOS transistors 8 and 39 are set such that theleakage current of P channel MOS transistor 8 immediately after thepower-on is larger than the leakage current of N channel MOS transistor39.

Therefore, nodes N1 and N2 attain an L level and an H level,respectively, immediately after the power-on. Thereafter, as powersupply potential VDD increases, the on current of P channel MOStransistor 2 increases, so that potential V1 of node N1 increases. Whenpotential V1 of node N1 exceeds the threshold potential VTN of inverter35, the potential of node N2 is lowered from an H level to an L level,and signal POR# is raised from an L level to an H level. In other words,potential V1 of node N1 is determined by the current drivingcapabilities of P channel MOS transistor 2 and N channel MOS transistor7, regardless of the rising speed of power supply potential VDD.Therefore, even if power supply potential VDD is slowly raised up,signal POR# remains at an L level for a prescribed time. Thus,malfunction of the semiconductor integrated circuit device is prevented.

In the modification shown in FIG. 5, resistance element 4 of the PORcircuit shown in FIG. 4 is replaced with a pull-down circuit 10.Pull-down circuit 10 includes an N channel MOS transistor 11, aresistance element 12 and a capacitor 13. N channel MOS transistor 11 isconnected between the line of power supply potential VDD and node N1,and has its gate connected to the line of power supply potential VDD viaresistance element 12 as well as to the line of ground potential GND viacapacitor 13.

During the time period in which power supply potential VDD is applied,capacitor 13 is charged to power supply potential VDD. During the timeperiod in which potential V1 of node N1 is at an H level, a leakagecurrent does not flow in N channel MOS transistor 11. Thus, the currentconsumption is reduced compared to the case of the POR circuit of FIG. 4where the leakage current flows through resistance element 4. When theapplication of power supply potential VDD is stopped, the charges incapacitor 13 are gradually discharged via resistance element 12 to theline of power supply potential VDD, and correspondingly, the gatepotential of N channel MOS transistor 11 gradually decreases. At thistime, N channel MOS transistor 11 is in an on state, so that the chargeson node N1 are discharged via N channel MOS transistor 11 to the line ofpower supply potential VDD. Thus, the potential V1 of node N1 becomes0V.

Referring to FIG. 6 illustrating the effects of the POR circuit of FIG.5, when the application of power supply potential VDD is stopped at agiven time, the potential of the line of power supply potential VDDstarts to decrease with time. Without pull-down circuit 10, it takes along time until the potential V1 of node N1 becomes 0V, and therefore,if power supply potential VDD is switched on again before potential V1reaching 0V, the semiconductor integrated circuit device is likely tomalfunction. Conversely, with pull-down circuit 10, potential V1 of nodeN1 reaches 0V quickly, and therefore, even if power supply potential VDDis switched on again afterwards, malfunction of the semiconductorintegrated circuit device is unlikely to occur.

In the modification shown in FIG. 7, pull-down circuit 10 of the PORcircuit shown in FIG. 5 is replaced with a pull-down circuit 14.Pull-down circuit 14 is identical to pull-down circuit 10 except thatthe resistance element 12 is replaced with a P channel MOS transistor15. P channel MOS transistor 15 is connected between the line of powersupply potential VDD and the gate of N channel MOS transistor 11, andhas its gate receiving ground potential GND. In this modification,again, the effects the same as in the POR circuit of FIG. 5 can beaccomplished.

In the respective POR circuit shown in FIGS. 1-7, power supply potentialVDD is divided by resistance value R2 of P channel MOS transistor 2 andresistance value R3 of N channel MOS transistor 3 before being appliedto inverter 35. However, if the gate lengths and/or the thresholdvoltages of MOS transistors 2, 3 vary due to variation in manufacturingprocesses, resistance values R2, R3 of MOS transistors 2, 3 will vary,thereby altering the threshold voltage Vres of the POR circuitconsiderably.

Thus, in the modification shown in FIG. 8, resistance elements 16, 17are added to the POR circuit of FIG. 7. Resistance element 16 isinterposed between the drain of P channel MOS transistor 2 and node N1.Resistance element 17 is interposed between node N1 and the drain of Nchannel MOS transistor 3. Resistance elements 16, 17 are each formed ofa diffusion resistance layer, a polycrystalline silicon layer or thelike. Resistance elements 16, 17 are formed of the same material to havethe same width, and their lengths determine their respective resistancevalues R16, R17. Resistance values R16, R17 of resistance elements 16,17 are set sufficiently larger than resistance values R2, R3 of MOStransistors 2, 3 at the time when power supply potential VDD reaches thethreshold potential Vres of the POR circuit. Therefore, thresholdvoltage Vres of the POR circuit becomes equal to VTN(R16+R17)/R17. Thus,in this modification, resistance values R16, R17 of resistance elements16, 17 are less likely to be affected by the process variation comparedto resistance values R2, R3 of MOS transistors 2, 3. Thus, it becomespossible to stabilize threshold voltage Vres of the POR circuit.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A power on reset circuit incorporated in asemiconductor device and generating a reset signal for resetting saidsemiconductor device when power is turned on, comprising: an inverterdriving said reset signal to an activated level in response to receptionof a power supply potential and a reference potential, and driving saidreset signal to an inactivated level in response to a potential of aninput node of the inverter exceeding a predetermined thresholdpotential; a first resistance element having one electrode receivingsaid power supply potential and another electrode connected to the inputnode of said inverter; and a first transistor of a first conductivitytype having a first electrode receiving said reference potential and asecond electrode connected to the input node of said inverter, andrendered conductive in response to said reset signal attaining theactivated level; wherein said first resistance element includes a secondtransistor of a second conductivity type having a first electrodereceiving said power supply potential, a second electrode connected tothe input node of said inverter and an input electrode receiving saidreference potential; wherein said inverter includes a third transistorof the second conductivity type having a first electrode receiving saidpower supply potential, a second electrode connected to an output nodeof said inverter and an input electrode connected to the input node ofsaid inverter, and a fourth transistor of the first conductivity typehaving a first electrode receiving said reference potential, a secondelectrode connected to said output node and an input electrode connectedto said input node, said predetermined threshold potential being madeapproximately equal to a threshold potential of said fourth transistor;a seventh transistor of the first conductivity type having a firstelectrode and an input electrode both receiving said reference potentialand a second electrode connected to the input node of said inverter; andan eighth transistor of the second conductivity type having a firstelectrode and an input electrode both receiving said power supplypotential and a second electrode connected to the output node of saidinverter.